Disc drives are commonly used in workstations, personal computers, laptops and other computer systems to store large amounts of data in a form that can be made readily available to a user. In general, a disc drive comprises a magnetic disc that is rotated by a spindle motor. The surface of the disc is divided into a series of data tracks. The data tracks are spaced radially from one another across a band having an inner diameter and an outer diameter.
Each of the data tracks extends generally circumferentially around the disc and can store data in the form of magnetic transitions within the radial extent of the track on the disc surface. An interactive element, such as a magnetic transducer, is used to sense the magnetic transitions to read data, or to transmit an electric signal that causes a magnetic transition on the disc surface, to write data. The magnetic transducer includes a read/write gap that contains the active elements of the transducer at a position suitable for interaction with the magnetic surface of the disc. The radial dimension of the gap fits within the radial extent of the data track containing the transitions so that only transitions of the single track are transduced by the interactive element when the interactive element is properly centered over the respective data track.
The magnetic transducer is mounted by a head structure to a rotary actuator arm and is selectively positioned by the actuator arm over a preselected data track of the disc to either read data from or write data to the preselected data track of the disc, as the disc rotates below the transducer. The actuator arm is, in turn, mounted to a voice coil motor that can be controlled to move the actuator arm across the disc surface.
A servo system is typically used to control the position of the actuator arm to insure that the head is properly centered over the magnetic transitions during either a read or write operation. In a known servo system, servo position information is recorded on the disc surface between written data blocks, and periodically read by the head for use in a closed loop control of the voice coil motor to position the actuator arm. Such a servo arrangement is referred to as an embedded servo system.
In modern disc drive architectures utilizing an embedded servo, each data track is divided into a number of data sectors for storing fixed sized data blocks, one per sector. Associated with the data sectors are a series of servo sectors, generally equally spaced around the circumference of the data track. The servo sectors can be arranged between data sectors or arranged independently of the data sectors such that the servo sectors split data fields of the data sectors.
Each servo sector contains magnetic transitions that are arranged relative to a track centerline such that signals derived from the transitions can be used to determine head position. For example, the servo information can comprise two separate bursts of magnetic transitions, one recorded on one side of the track centerline and the other recorded on the opposite side of the track centerline. Whenever a head is over a servo sector, the head reads each of the servo bursts and the signals resulting from the transduction of the bursts are transmitted to a microprocessor within the disc drive for processing, for example.
When the head is properly positioned over a track centerline, the head will straddle the two bursts, and the strength of the combined signals transduced from the burst on one side of the track centerline will equal the strength of the combined signals transduced from the burst on the other side of the track centerline. The microprocessor can be used to subtract one burst value from the other each time a servo sector is read by the head. When the result is zero, the microprocessor will know that the two signals are equal, indicating that the head is properly positioned.
If the result is other than zero, then one signal is stronger than the other, indicating that the head is displaced from the track centerline and overlying one of the bursts more than the other. The magnitude and sign of the subtraction result can be used by the microprocessor to determine the direction and distance the head is displaced from the track centerline, and generate a control signal to move the actuator back towards the centerline.
Each servo sector also contains encoded information to uniquely identify the specific track location of the head. For example, each track can be assigned a unique number, which is encoded using a Gray code and recorded in each servo sector of the track. The Gray code information is used in conjunction with the servo bursts to control movement of the actuator arm when the arm is moving the head in a seek operation from a current track to a destination track containing a data field to be read or written.
The head structure also includes a slider having an air bearing surface that causes the transducer to fly above the data tracks of the disc surface due to fluid currents caused by rotation of the disc. Thus, the transducer does not physically contact the disc surface during normal operation of the disc drive to minimize wear at both the head and disc surface. The amount of distance that the transducer flies above the disc surface is referred to as the “fly height.” By maintaining the fly height of the head at an even level regardless of the radial position of the head, it is ensured that the interaction of the head and magnetic charge stored on the media will be consistent across the disc.
A disc drive is usually made up of multiple discs, or platters, each of which uses two heads to record and read data, one for the top surface of the platter and one for the bottom surface of the platter. The heads that access the platter surfaces are locked together on an assembly of rotary actuator arms. As a result, all the heads move in and out together so that each head is always physically located at the same track number.
Because of this arrangement, often the track location of the heads is not referred to as a track number but rather as a cylinder number. A cylinder is the set of all tracks that all the heads are currently located at. For example, if a disc drive has four platters, it would have eight heads, and cylinder number 400, for example, would be made up of the set of eight tracks, one per platter surface, at track number 400.
Each cylinder contains a number of tracks, each accessible by one of the heads on the drive. To improve efficiency, the disc drive will normally use all of the tracks in a cylinder before going to the next cylinder when doing a sequential read or write since this saves the time required to physically move the heads to a new cylinder. Switching between heads within a cylinder requires a certain amount of time, called the head switch time. This is usually less than the track switch time, and is usually on the order of 100 microseconds to 1 millisecond.
The head switch time is calibrated during the initial certification process of the disc drive. That is, when the disc drive is manufactured, a certification process is performed in which firmware is loaded into the disc driver and various tests are performed on the disc drive to optimize its performance. It is during this certification process that the head switch timing is measured at different locations on the discs so that the disc drive will know how to adjust the timing before any head switch is performed. This assures that no address mark, i.e. a timing reference that identifies the beginning of a servo sector, will be missed during a head switch and that the driver will always know which servo sector it is on.
The address mark is the basis for timing the detection of the other fields of a servo sector, including the Gray code and positioning bursts. Thus, an address mark (AM) detect search window is established for the disc drive which indicates a period of time in which an address mark is expected to be detected. This AM detect search window needs to be kept small so that the probability of a false detection of an AM is kept to a minimum. This means that the variation in timing seen when doing a head switch must be kept small.
Recently it has been discovered that the head switch timing can change considerably from the head switch timing measured during the certification process due to external influences on the disc drive. For example, external forces on the disc drive, such as during mounting of the disc drive in a computing device, may cause flexing of the base plate of the disc drive such that the disc drive is distorted from the state it was in during the certification process. In addition, some disc drives are susceptible to temperature variations which may also cause the base plate of the disc drive to distort. Such distortions cause the timing of the disc drive to be off from the optimized timing determined during the certification process.
In order to compensate for such distortions in timing, the AM detect search window could be increased so that the likelihood that an AM is missed is reduced. However, increasing the AM detect search window increases the likelihood of a false detection of an AM. Moreover, the AM detect search window would need to become excessively large to cover all of the variation that would be seen by the disc drive due to external factors and thus, false detection of an AM would almost be certain.
The present invention provides a solution to this and other problems, and offers other advantages over previous solutions.